A solid-state 13C NMR analysis of molecular dynamics in aramide polymers

The local dynamics of aromatic cores was analyzed for a homologous series of polyamides in the solid phase incorporating phenyl, biphenyl and naphthyl groups. Preliminary wide-line and spin-relaxation variable-temperature 1H NMR measurements revealed the presence of thermally activated molecular motions for each polymer studied. A number of 13C NMR experiments were then implemented to further clarify the nature and extent of such motions. These included 1H-13C 2D separate-local-field measurements, whose line shapes revealed that motions involved for all cases a superposition of states. These could in principle be associated with rigid and mobile populations in these semi-crystalline aramides, a model that yielded a proper description of the spectra at all temperatures. To further probe this model the relaxation behavior of the aramides' 13C spins was monitored in the rotating frame as a function of temperature, in both the presence and absence of homonuclear 1H-1H decoupling. The variations observed in these measurements evidenced a thermally activated, relatively broad distribution of motional rates in the polymers. Editing the 2D local-field data according to the 13C relaxation also supported this heterogeneous dynamic model. The mechanism underlying this behavior and implications towards the 13C analysis of motions in aramides in particular and complex polymers in general, is briefly discussed.